* Convert TypeSet fields to sets; Add BitSet<T> type to rust; Encode ValueTypeSets using BitSet; (still need mypy cleanup) * nits * cleanup nits * forgot mypy type annotations * rustfmt fixes * Round 1 comments: filer b2, b4; doc comments in python; move bitset in its own toplevel module; Use Into<u32> * fixes * Revert comment to appease rustfmt